Solid-state organic light emitting diode (OLED) image displays are of great interest as a superior flat-panel display technology. These displays utilize current passing through thin films of organic material to generate light. The color of light emitted and the efficiency of the energy conversion from current to light are determined by the composition of the organic thin-film material. Different organic materials emit different colors of light. However, as the display is used, the organic materials in the device age and become less efficient at emitting light. It is useful, therefore, to provide a means to correct the light output of an OLED device over time. It is also useful to detect the ambient illumination incident on an OLED display and to modify the brightness of the OLED display in response to the ambient illumination.
OLED devices are conventionally either top-emitting or bottom-emitting. An OLED display is constructed upon a substrate by depositing an electrode upon the substrate, organic light emitting materials over the first electrode, and a second electrode above the light emitting materials. A cover is used to encapsulate and protect the device. Light is emitted by the application of a current from one electrode to another passing through the organic light emitting materials. A bottom emitting device emits light through the substrate and first electrode which must both be transparent. The second electrode in a bottom emitting device may be either transparent or reflective. A top-emitting device emits light through the cover and second electrode which must both be transparent. The first electrode in a top-emitting device may be either transparent or reflective.
OLEDs emit light in every direction. A portion of the light is emitted directly toward the front of the display device: through the substrate (for a bottom-emitter device) or the encapsulating cover (for a top-emitter device). A similar portion of the light is emitted toward the back of the display device and may be either absorbed or reflected by the electrode or other layers behind the organic layers. If the portion of light emitted toward the back is reflected, it can pass through the organic layers again and be emitted through the front of the display, thereby increasing the brightness of the display. If the portion of the light emitted toward the back is absorbed, it is absorbed and lost, thereby reducing the light emitted by one half.
Because OLED materials age, it is known to calibrate OLED display devices through the use of external sensors which measure the light output from the display device and construct a calibration table for use by the device to correct for aging. See for example U.S. Pat. No. 5,371,537, issued Dec. 6, 1994 to Bohan et al. This approach has the problem that the sensor device obscures the display during the calibration and is not capable of providing real time operation. Moreover, these approaches are not useful for correcting uniformity variations among individual pixel display elements.
Alternative methods utilize a light sensor integrated with the light emitting elements of the display themselves. For example, U.S. Pat. No. 6,489,631, Dec. 3, 2002, entitled “Light-Emitting Matrix Array Display Devices with Light Sensing Elements” describes the integration of a photosensitive device with an electro-luminescent pixel element. The light sensing elements each comprise a gated photosensitive thin-film device such as a TFT structure having a semiconductor layer with contact regions laterally spaced on the substrate and separated by a gate controlled region. A part of the associated display element extends over the gate controlled region with an electrode of the display element serving as the gate of the photosensitive device, thereby ensuring good optical coupling between the display element and the photosensitive device. This arrangement fails to provide adequate sensitivity for sensing ambient illumination in an actual device and does not provide feed-back on the OLED light output uniformity.
There is a need therefore for an improved OLED display having integrated photosensors.